The present invention relates to a method of printing an image on a substrate. The invention can be used in lithography, information recording, chemical milling and printing, but is particularly useful for producing printed circuit boards, and is therefore described below with respect to this application. It will be appreciated, however, that the invention could advantageously be used in many other applications.
Printed circuit boards (PCBs) are constructed of a substrate of insulating material, such as epoxy glass, having an electrically conductive pattern or net printed on one or both faces. Many techniques have been developed for printing the conductive pattern. The techniques commonly used today start with a substrate having a copper layer on one or both faces. In the production line, the board is coated with a photoresist film, namely a protective film sensitive to light. The photoresist film is exposed to ultraviolet light through a phototool or mask which is photographically constructed according to the desired pattern to be printed, producing a latent image of the pattern in the exposed areas of the photoresist film. The exposed portions of the photoresist film may be rendered less soluble or more soluble. The portions of the photoresist film which are more soluble are removed to bare the copper thereunder. According to some techniques, the so-bared copper is metal-plated; then the remainder (insoluble portion) of the photoresist protective film is removed to bare the copper thereunder, and the so-bared copper portions are completely removed by etching. According to other techniques, the portions of the copper bared by removing the soluble portions of the photoresist are etched, and then the insoluble portions of the photoresist are removed.
The foregoing process for producing printed circuit boards, as well as IC integrated circuit wafers or the like, thus requires the preliminary preparation of the phototools. The preparation of phototools takes considerable lead time, which is not suitable for modern manufacturing demands requiring just-in-time manufacturing techniques. The use of phototools also requires high investment in materials, machines, special environmental storage, and man-power to operate the phototool department. In addition, the large number of steps, and particularly the large number involving human operations, significantly affect the yield.
Because of the foregoing drawbacks in utilizing phototools, efforts are being made for developing new imaging techniques. Particularly promising are direct imaging techniques wherein the pattern is directly imaged on the photoresist by using a beam, such as an X-ray beam, an electron beam, an ion beam, or a laser beam. The beam is operated by using computer-controlled means to provide a sequence of preprogrammed on/off modulating writing signals as the beam is caused to scan the photoresist.
In the basic direct imaging process (as well as in the conventional flood exposure process), ultra- violet or visible radiation is used to expose photographically the photoresist and to activate a photochemical reaction which produces a latent image. The latent image is developed to produce a protective layer pattern. Since the photoresist is sensitive to light (ultraviolet or visible), a safety light must be used from the time the photoresist layer is applied until the protective layer pattern is produced and stabilized.
Another direct-imaging technique is known (e.g. as described in U.S. Pat. No. 4,013,466) in which a visible (blue) flood exposure is used to activate a photochemical reaction in the photoresist, as in the prior art of flood and laser direct-imaging. Here, however, the conventional phototool is replaced by an "eraseable phototool" in the form of a liquid crystal (LC) exposed to an infrared laser to induce a phase transition by the local heating produced by the laser; see for example U.S. Pat. No. 4,013,466. The liquid crystal thus serves as an "eraseable phototool" in place of the regular phototool or photo mask. However, this process also requires the use of a safety light. Moreover, because of the sensitivity of the liquid crystal to ultraviolet light, this process requires the use of a special expensive photoresist, and also only visible light.
Still another direct-imaging technique is described by V. M. Andreev, et al., Autometria, No. 3, 1990, pp. 102-105. In this method, an insulating substrate is coated with a film of a complex salt of copper hypophosphite [Cu(H.sub.2 PO.sub.2).sub.2 ], or its aminoammonia complexes, and is thermally decomposed when subjected to a laser beam. This results in the formation of fine, dispersed copper particles which exhibit catalytic activity in electroless copper plating processes. This method thus concerns depositing copper particles to be used as a catalytic agent for a subsequent electroless copper plating process, and not the formation of a protective layer for use in subsequent etching or plating processes involved in printed circuit techniques.